The Cedara dairy herd of 100 lactating Friesian cows was randomly subdivided into three treatments. A mineral supplement comprising a standard salt + dicalcium phoshate mixture (Treatment 1), the standard + 5% of a commercial trace-mineral premix (Treatment 2) and the standard mix + 10% MgO (Treatment 3) was fed at l30g/cow/day. Inter-calving periods were significantly (P<0,05) reduced from 394 days in Treatment 1 to 373 in Treatment 3 while the services to conception were reduced from 1,94 to 1,54 by magnesium supplementation.